The present invention pertains generally to highly accurate measurement of forces, and more particularly to measurement of dynamic forces which are transmitted through structural or machine elements.
Conventional force sensing technology (CT) pursues the maximization of mechanical reasonant frequencies in order to achieve as flat a frequency response for as high a frequency range as possible before the resonant peak produces unacceptable amplification of the forces being input to the measurement system, as sensed and reported by system load cells. Unfortunately, the measurement bandwidth achievable is severely limited with this strategy. Most measurement tranducers are used to only {fraction (1/10)}th of their resonant frequencies. Such a practice would, for example, limit High Speed Uniformity (HSU) tire test machines to only about a 20 Hz bandwidth. Most tire test engineers consider measurements to xc2xe the resonant frequency of an HSU machine to be useful; however, this does amplify forces measured at such a frequency by a factor of over 2x, which may not be known by the user of the data.
At low excitation frequencies (e.g., less than {fraction (1/10)}th the first resonance of the system) both CT and dynamic force measurement (DFM) methods produce similar results. This is generally in the 20 to 50 Hz range. Above that, the two methods produce widely divergent responses to identical inputs. The narrow bandwidth produced by CT methods used to be acceptable; however, today""s automotive engineering demands measurement bandwidths that are more than ten times those which conventional technology can produce. Many engineers are now searching for bandwidth improvement, usually through Fast Fourier Transform (FFT) and Frequency Response Function (FRF) measurements, requiring postprocessing of experimental data to compensate for the resonance effects.
Load cells are displacement devices with a stiffness of k force units per unit of displacement so that an equation of motion for the system shown in FIG. 17 is:                                           ∑                          xe2x80x83                        ⁢            F                    =                                    ma              ⁢                              
                            -              kx              +                              f                ⁡                                  (                  t                  )                                                      =            mx                          ⁢                  
                ⁢                              mx            +            kx                    =                      f            ⁡                          (              t              )                                                          Eq        .                  xe2x80x83                ⁢        1            
Damping is ignored in these equations, but may be added if required as:
m{umlaut over (x)}+c{dot over (x)}+kt =f(t)xe2x80x83xe2x80x83Eq. 2
where m{umlaut over (x)} may be estimated by exciting the system with a known transient impulse f(t) and measuring {umlaut over (x)} with an accelerometer, then multiplying it by the mass m. kx is the load cell signal. C{dot over (x)} may be estimated by integrating the {umlaut over (x)} signal to get {dot over (x)}, then multiplying by the parameter c so that the sum of the three signals add to produce the known input f(t).
This process may be typically simplified by ignoring the damping term cx and summing only m{umlaut over (x)} and kx to equal f(t), which works very well. The mass m may be estimated (evaluated) by exciting the system with a transient impulse f(t) and measuring {umlaut over (x)} multiplying by an adjustable constant m, and summing the resulting signal with the load cell signal to null the output during the ring-down period following cessation of application of the transient force f(t).
This general technique is called inertia compensation and has been used in the past to measure applied impact forces during crash tests and other situations that involve accelerated load cells, such as in load cells attached to an automobile wheel that experiences suspension motions during vehicle operation. This technique has not generally been extended to stationary test machines with nominally motionless load cells. Such machines measure input forces through attached masses, as well; however, since the masses are assumed to be motionless (unaccelerated) they are assumed to generate no inertial forces. However, this assumption is not true and the general result of this error is to limit the frequency range of valid measurement by the resonant frequency of the load cell supported mass. As discovered by the inventor and explained herein, the effects of the resonant frequency may be removed from such stationary machines through the above addition of the ineitia force to the load cell force to yield the input force.
The dynamic force measurement methods and systems of the present invention provide the first direct measurement technique capable of the desired bandwidth expansion. Dynamic force measurement devices constructed in accordance with the invention are capable of measuring machine resonant forces which may then be added directly to the load cell forces, thus canceling their effects therein, leaving the test specimen output forces for direct real-time recording during the test, rather than during a post processing session involving Fast Fourier Transform (FFT) and inverse Frequency Response Function (FRF) calculations. Furthermore, the process does not depend upon measurement system linearity. Losses and nonlinear stiffnesses are included in the measurements and fall away in the analysis, thus dispensing with the concerns for both linearity and post processing time and inconvenience.
The invention further provides the ability to select output data as load cell, dynamic force measurement, or both for comparison purposes. This function enables the comparison of data acquired from similar test machines built at different times by different makers, or which may produce different results due to unique resonant frequency signatures of each machine. The methods and apparatus of the invention improve the measurement reproducibility of same-specimen output data on all such machines, making existing test systems upgradable to meet the latest test requirements by allowing wide measurement bandwidth performance. The same high performance test results are also available from newly built low-cost test machines.